Microsoft's Windows 8 operating system went on sale on October 26th, and the company has just announced it has already sold four million upgrade copies. At a keynote address in Seattle today, Microsoft CEO Steve Ballmer revealed the first Windows 8 statistics — four million upgrade copies over the first weekend of sales. Ballmer mentioned the launch of Surface, but did not reveal any specific sales figures for that particular device.

Of course, considering the vast size of Microsoft's install base, these numbers are altogether different, rendering it somewhat difficult to compare the two.

Still, Windows 8 is an enormous departure from the typical Windows design language of the past, and will likely cause a slowdown in adoption. Windows is, also, not typically installed on a first-day-out basis, but is often held up by systems administrators or unused by otherwise unaware families.

In the Windows universe, from my experience, a new operating system is often seen only when buying a new PC. Few non-technical people upgrade their PCs unless they are prompted to do so.

For all of this, however, it's still rather early to be judging Windows 8's performance. Although some have been quick to ridicule these numbers, they do not sound awful to me. But only time will tell.